The materials in question are used on a large scale for the production of mineral wool which is in particular intended for thermal and acoustic insulation.
On one hand, the reasons why these materials are chosen can be found in their low costs, and on the other hand in their properties, especially their good resistance against high temperatures. Their production, however, creates specific problems. These problems particularly stem from the conditions in which these materials are workable.
Their high melting temperatures already present a difficulty in itself. The melting temperature is the temperature to which the raw materials have to be heated to guarantee melting. Furthermore, where production is concerned, it is the temperature above which the material must be kept in order to flow through the fiberizing device.
Another particularity discerning these materials from the glasses traditionally used for fiber production is that, as a rule, they are highly fluid at temperatures very close to their liquidus temperatures. The resulting difficulties shall be explained in the following.
Also due to the required high temperatures, the devices getting into contact with the materials to be fiberized are subject to very intensive corrosion. Operational lifetime of these devices presents a problem even with conventional glasses. The problem becomes even more critical with high liquidus materials.
Hitherto, the above-mentioned difficulties meant that only certain fiberizing techniques could be applied with the materials in question. There are essentially two kinds of techniques: those employing centrifuging, or spinning off, of the molten mineral material, and those where the material is fed through a stationary nozzle and attenuated into fibers by gas flows often accelerated to supersonic speeds (blast drawing method).
For techniques applying a fixed nozzle, it is necessary to utilize a nozzle which is able to resist the attack of the molten mineral material. Usually, these are platinum nozzles able to withstand these attacks even at such high temperatures. Production capacity of each nozzle, however, is limited. In addition, the attenuating gas flows required generate comparatively high energy costs.
Techniques employing centrifuging, or spinning off, allow considerable production quantities per unit. Those are techniques summarized under the generic term "external centrifuging", in order to indicate that the molten mineral material remains outside the spinner. The molten mineral material is either applied to the front surface of a disk or to the peripheral surface of a cylindrical rotor, or a plurality thereof. An advantage of these techniques is the simplicity of the parts of the device entering into contact with the molten mineral material. With respect to this relative simplicity, the parts in question, in particular the spinner rims, are relatively cheap and therefore permit exchange within relatively short time spans. The proportion of material costs figuring among total production costs remains relatively low. The fact that these device parts are subject to intensive wear upon contact with the molten mineral material does therefore not turn out to be an obstacle.
The main disadvantage of mineral fiber production by external centrifuging lies in the fact that the properties of the final product, at equal fiber quantities, are inferior to those of glass wool which is mainly produced by so-called "internal centrifuging".
In external centrifuging, the material flows onto the spinning wheels and is flung off them as a multiplicity of droplets. The fiber apparently forms once it is flung off, between the surface of the spinner and the droplet drawing the fiber after it. It is obvious that with such a fiberizing mechanism, a considerable portion of the spun-off materials remains in the form of unfiberized particles. Their proportion can be as high as 40 weight percent for particle sizes of more than 100 .mu.m of the starting material. Although several methods are available for separating the unfiberized particles, the finished mineral wool is never entirely free of such particles which at best are of no use, and very much of a nuisance for particular applications.
It should be pointed out that drop formation is not exclusively the result of external centrifuging, but depends also on the rheological characteristics of the materials in question. Materials processed according to the invention generally have comparatively low viscosities, even at temperatures only slightly above liquidus temperature. The molten mineral material, which is relatively fluid, is difficult to fiberize as the filaments have a tendency to break and to form drops or beads. In a way, the technique of external centrifuging relies on this tendency, however without eliminating its disadvantages.